Where is Task Manager on a Mac?

Two things cover what Task Manager does:

• ⌥⌘Esc opens Force Quit Applications, a short list of running apps with a button to end the one that stopped responding. This is the Mac's Ctrl+Alt+Del.
• Activity Monitor, in Applications → Utilities, or via ⌘Space and typing "act". It lists every process with CPU, memory, energy, disk and network tabs, and its stop button (the x in the toolbar) ends any of them. The View menu can show all processes, and the Dock icon can show a live CPU graph.

A frozen app on a Mac shows the spinning rainbow cursor. Give it a few seconds first; if it stays, ⌥⌘Esc, pick the app, Force Quit. Unsaved work in that app is lost, as on Windows.

Startup programs are not in Activity Monitor. They live in System Settings → General → Login Items & Extensions.